This invention relates to pulse Doppler diagnostic systems, and is more particularly directed to improvements in such systems enabling increased accuracy, facility of use, signal-to-noise ratio, and adaptability to different modes of operation of such systems. While the following disclosure is directed specifically to the use of the invention in medical applications such as monitoring blood flow, it will be apparent that the concepts of the invention are adaptable for other applications, and it is intended herein that the scope of the invention include such other applications.
In the past, medical systems have been provided that employ pulse Doppler ultrasound for non-invasive cardiac diagnosis, such as monitoring of blood flow in vessels and arteries and determining pulmonary arterial pressure or hypertension. Such systems employ echo, continuous wave or pulse Doppler ultrasound signals. Such systems are disclosed, for example, in U.S. Pat. No. 4,058,001, Waxman, U.S. Pat. No. 4,137,777, Haverl, U.S. Pat. No. 4,205,555, Hashiguichi, U.S. Pat. No. 4,242,911, Martin, U.S. Pat. No. 4,103,679, Aronson, U.S. Pat. No. 4,141,347, Green et al., U.S. Pat. No. 3,802,253, Lee, U.S. Pat. No. 4,097,835, Green, U.S. Pat. No. 4,313,444, Glenn, U.S. Pat. No. 4,318,413, Iinuma, U.S. Pat. No. 4,387,720, Miller, U.S. Pat. No. 4,390,025, Takemura, et al, U.S. Pat. No. 4,398,540, Takemura et al and U.S. Pat. No. 4,407,293, Suarez, Jr., et al.
Using the Doppler principle, pulse and continuous wave ultrasound are presently being used in medicine to make non invasive measurements of peripheral and central cardiovascular blood flow velocities. Other ultrasound measurements such as cardiac valve movement velocities, valve time intervals, and contractual and relaxation periods of the cardiac chambers provide useful diagnostic information in the assessment of cardiovascular disease. The velocity measurements are made by processing the returned "echo" signals for a change in frequency. This Doppler-shifted frequency, which is directly related to the velocity of the reflector, is usually in the audible range and contains useful diagnostic information simply by listening. However, if the velocity and its direction are to be known and recorded, further electrical processing is necessary.
Flow velocity and direction information can be obtained by comparing the received signal frequency with the transmitted frequency and the transmitted frequency shifted 90 degrees in phase. Using tis technique, two Doppler shift signals are obtained which are equal in frequency but different in phase by 90 degrees according to velocity direction. These signals are commonly referred to as quadrature audio. When presented to the quadrature audio demodulator, if, for instance, the first channel is leading the second channel by 90 degrees, i.e., flow "toward the probe", the analog voltage at the output would be a positive value linearly related to the Doppler frequency. If, on the other hand, flow is away from the probe, the phase relationship will be reversed and the analog voltage at the output would have negative value in relation to frequency.
In the past, Doppler-shifted audio signals have been processed for visual inspection by fast Fourier transforms, Kay sonograms and direct frequency-to-voltage conversion by "zero crossing rate meter". All of these techniques have their strong and weak points. The zero crossing rate meter is simply a low-cost frequency-to-voltage converter, wherein the conversion is made by integrating pulses of constant amplitude and width which are generated when the Doppler frequency signal crosses a zero voltage reference.
Three common faults of the zero crossing rate meter are as follows: (1) false counts can occur when noise is present, which may cause extra crossings, (2) counts can be missed if a low frequency component is present, allowing the higher frequencies to ride above or below the zero level (riding high syndrome), (3) unpredictable phase shift between the quadrature channels can occur resulting in a loss of flow direction information. These conditions are the rule rather than the exception when processing back- scattered signals from deep vessels. The introduction of hysteresis and filters into the circuit may lessen the noise problems but introduce phase errors that degrade the important direction determination.